The present invention relates to thermoplastic foams and a process for producing thermoplastic foams using a carbon dioxide blowing agent. More particularly, the invention relates to a process for producing foam using a carbon dioxide blowing agent whereby certain additives are used to produce foams of improved quality.
Thermoplastic foams are generally produced in a tandem extrusion process (two extruders in series). The first extruder melts thermoplastic polymer to produce a polymer melt. A high-pressure metering pump delivers a blowing agent to the polymer melt near the end of the first extruder where mixing is initiated prior to entering the second extruder where further mixing and cooling of the blowing agent and polymer melt occurs. After exiting the second extruder, the polymer melt passes through and becomes a foam structure at a die, generally an annular die. The foam structure, generally in the shape of an annular tube, is then stretched over a forming mandrel. The annular tube coming off the mandrel is slit and opened to form a sheet which is then gathered on one or more rolls. The rolled foam sheet is typically aged for a predetermined period of time and then thermoformed into a foamed article, e.g., a packaging tray.
Blowing agents which are most commonly used at present include aliphatic hydrocarbons, e.g., C.sub.3 -C.sub.6 alkanes, and fully or partially halogenated hydrocarbons, e.g., chlorinated and/or fluorinated hydrocarbons. Such conventional blowing agents possess or cause one or more of the following undesirable characteristics: pollution, potential damage to the ozone layer, flammability, poor thermoforming characteristics, brittleness, high cost, and the need for a prolonged aging period to allow some or all of the blowing agent to diffuse from the foam sheet prior to thermoforming. For these reasons, alternatives to the foregoing blowing agents have been sought.
One such alternative is carbon dioxide, which avoids most or all of the foregoing undesirable characteristics of conventional blowing agents. However, due to the extreme volatility and high vapor pressure of carbon dioxide, it is a difficult blowing agent to use, often resulting in a foam sheet with ruptured cells, surface defects (e.g., cracks), and corrugation, i.e., thickness variations within the foam sheet which produce a rippled or wavy surface appearance. Each of the foregoing occurrences detrimentally affect the physical properties of the foam (e.g., strength) as well as the aesthetic qualities thereof.
Accordingly, there is a need in the art for an improved process for producing thermoplastic foam from a carbon dioxide blowing agent which produces a high-quality foam, i.e., one with fewer ruptured cells, surface defects, and corrugation.